I use riders in front of me as carrots! And when there is no one in front, I use oncoming riders to keep up my momentum. If you have aero bars you now have a target on your back!
The most precarious time when riding, is riding in a group. Even with people that you know, There are too many people out there that will focus just on keeping on your rear wheel and not looking ahead! You touch your brakes or swerve a little and instant crash!
It is hard enough riding with people that you know, so having a unknown rider on your wheel would be extremely bad!! You might not go down with a rear wheel contact but they sure as heck will!
I personally dont get wheel suckers, I refuse to! I will use every ounce of strength to ride them off , and they will eventually fade off in the distance. I secretly love destroying people it is how I train. Don't get caught, Ride down every jersey that you see! And if you are using aero bars, I will sit up, pretend that I am not breathing hard, give you a pleasent Good Morning and head nod as I pass you. While I am dying inside!
In short unkown people on your wheel BAD!!!
I'm not a bad person, I will give you the jersey off of my back. I will change your flat if I see you stranded, I have given up a tube a few times, I nodd at all oncoming riders and always say good morning. But try to ride on my back and I will take it as an insult, and that I am not riding hard enough! You will be dropped!!